I recently installed Ubuntu 14.04 on Lenovo Idea Pad S12. I am not able to log in as the admin nor the guest. When trying to open Terminal, I only get black-white-blue squares and have to restart the computer. I tried 2 different bootable USB with a different version of Ubuntu and also Linux Mint. I only get a black screen with a flashing underscore. I can't log in, can't insert any commands and can't boot from a USB. Please help.
Try a liveCD. If you can't boot even a livecd, you need to go to BIOS/UEFI menu and change the boot device order. Choose CD/DVD driver first, then USB or usb-zip, then HDD/SSD. Save changes, exit BIOS/UEFI, and reboot. On some laptops it is enough to just press ESC key at startup and a boot menu will appear asking you what device to boot from.